Zipper

It’s not an option written into the pattern as it requires redrafting the front panels.
You can either make it a single, non overlapping closure with a zipper, or insert a zipper into the underlying facing and keep the buttons over the top.
Deb describes and shows the second option very well on her blog. Link : http://sewingstashchallenge.blogspot.com.au/2014/10/school-days-jacket.html